Production of cyclohexane



Patented Aug. 22, 1944 PRODUCTION OF CYCLOHEXANE Marston L. Hamlin. ynbrook, N. Y., assignorto Allied Chemical & Dye Corporation, New York, N. Y., a corporation or New York Application July 28. 1942, Serial No. 452,591

8 Claims.

This invention relates to the recovery of cyclohexane from mixtures thereof with benzene.

The invention finds particular application in the production of substantially pure cyclohexone from cyclohexane-benzene mixtures containing a major proportion of cyclohexane and the balance substantially ,all benzene, such as mixtures produced by hydrogenation of benzene to cyclohexane.

In United States application Serial No. 354,558, flied August 28, 1940, by Charles R. Clark, there is described a process for the production of pure benzene from benzene fractions comprising benzene and other hydrocarbons normally boiling from the mixture in the same range as the benzene. The process of that application involves and azeotropic distillation of the benzene fraction with a lower alcohol such as methanol, eth.. anol, normal and isopropyl alcohol, and the butyl and amyl alcohols. Methanol is especially efiective for the production of a benzene product having a high degree of purity. In the preferred process of that application non-aromatic hydrocarbons together with a small proportion of benzene are distilled oil in the form or azeotropes with methanol leaving behind the substantially pure benzene product. In batch operations with suitable fractionation a distillate may be obtained which contains from 4% to 6% of benzene at the start and a gradually increasing proportion as the benzene content oi the distilland becomes higher. The process cannot. however, be made to yield a non-aromatic hydrocarbon distillate substantially free from benzene under distillation conditions which it is practicable to employ in large scale recovery of benzene from oils containing the same.

The present invention is concerned with the recovery of cyclohexane substantially free from benzene, containing not more than 2% benzene by volume, from cyclohexane-benzene mixtures. I In accordance with the present invention, I have discoveredthat by subjecting mixtures of propyl alcohol, cyclohexane practically free from benzene maybe distilled oil. as an .azeotrope with lsopropyl .alcohol leaving behind practically the entire benzenecontent or the original mixture.

cyclohexane andbenzene to distillation with iso- This result was contrary to that which was to be i expected from a consideration of the relative boiling points of certain azeotropes. Th methanol azeotropes of benzene an cyclohexane have boiling points diflering, by. more than icenti grade degrees and theisopropyl alcohol .azeoclohexane, and in using these agents for the separation of cyclohexane-benzene mixtures under comparable conditions, distillates are initially obtained containing on the order of four to five times as much benzene as when using methanol. Thus, among the lower alcohols I have discovered lsopropyl alcohol stands alone as an effective agent for the productionof cyclohexane substantially free from benzene.

An important advantage of the process of the present invention over other azeotropic distillation processes for the production of benzenefree cyclohexane lies in thehighgcyclohexane content of the azeotropic distillate produced by employing isopropyl alcohol, For example, distillation with, acetone has been proposed as a method for separating cyclohexane and benzene. Whereas azeotropic distillation of cyclohexane by means of acetone requires vaporization of about 6 parts by volume of acetone for each 3 parts of cyclohexane recovered, the present process requires vaporization of only 2 parts of isopropyl alcohol for recovery of the same amount of cyclohexane. Thus, 1 volume of isopropyl a1- cohol is able to accomplish the work of 3 volumes oi! acetone. This not only effects a substantial heat economy, but also reduces the size or equipment necessary.

In the treatment of benzene-cyclohexane mixturescontaining a large proportion of cyclohexane from which it is desired to separate both components in a pure state, the process may be used to advantage for removing a .pure cycl hexane product and the residue may then be dis- .tilledwith methanol toproduce a pure benzene product. .In this way the separation may be ef- Jected with the consumption of substantially less heat than required ii the acetonedistillation is employed without previous treatment of the cyclohexane-benzene mixture. r I

Frequently. however, it is not necessary to tropes of benzene and cyclohexanc have boiling 63 separate pure benzene from such mixtures; ior

example, in treating benzene hydrogenation products it may be economical to recover most of the cyclohexane in pure form as described and recycle the benzene residue, containing a small amount of cyclohexane. through the hydrogenation step mixed with fresh benzene.

The process of the invention may be conducted in a continuous distillation system or in a batch still. The volume of isopropyl alcohol employed in the process should be at least two-thirds of the volume of cyclohexane to be recovered as benzenefrce product. This ratio is in volumes of the alcohol and cyclohexane as liquids. The ratio of alcohol to cyclohexane may be much greater. Thus, ratios of about 2 volumes of the alcohol to about 1 volume of the cyclohexane have been used but the excess alcohol merely remains in the distillation residue.

One mode of conducting the process of the invention batchwise is illustrated diagrammatically in the accompanying drawing.

The numeral l designates a still having an inlet 2 for isopropyl alcohol and an inlet 3 for benzenecyclohexane mixture. The still I has a heating element 4, a drain outlet 5, and a rectifying column G which may be of conventional type and may have any desired degree of rectification efficiency. The higher the degree of efllciency of the rectification column, the*,lower will be the proportion of reflux which it is necessary to return for a desired degree of rectification.

From rectifying column 5,;izapor conduit 1 and branch conduits la and lb ving valves 8 and 9 lead respectively to product condenser i and reflux condenser ll. Condenser II is connected to a vapor vent l2 and liquid reflux return line l3. Condenser I0 is connected by an outlet pipe I, having branches Ma and Nb provided with valves l and iii, to tanks or separators l1 and I8. Tank I1 is connected to a water supply line [9 controlled by valve 20. Pipe 2| connects tank I! with a withdrawal line 22 for aqueous isopropyl alcohol and a withdrawal line 23 for cyclohexane. A return line 24 is provided on tank l8 for recirculation of an intermediate benzene fraction back to still I. The lines are suitably provided with valves 25, 26, and 21 for control of flow therethrough.

In operation of this system for the production of substantially pure cyclohexane from a mixture of cyclohexane and benzene, the mixture is charged to still I via line 3, and isopropyl alcohol is charged to the still through pipe 2 in amount sufllcient to provide at least 2 volumes of the alcohol for every 3 volumes of cyclohexane which is to be distilled out of the cyclohexane-benzene mixture. Heat is supplied through heating element 4 and valves 8 and 9 are adjusted to provide the desiredrefiux ratio to the top of column 6.

During the early stages of the distillation valve i5 is maintained open to collect condensate in tank II. This valve is maintained open until, in order to maintain the desired distillation rate, it becomes necessary to permit benzene to pass into the distillate. Valve is then closed and valve I6 is opened to permit subsequent distillate to collect in tank It. Valve may now be opened to introduce suiiicient water to tank H to provide effective separation of alcohol as an aqueous lower layer. After this separation has been effected valve is opened for a limited period to discharge the aqueous layer to suitable storage. The cyclohexane may be washed with one or more portions of fresh water, if desired, and the wash water may be discharged at 22 and added mixture,

" termediate fraction.

- centrate.

desired further treatment. removing the isopropyl alcohol in the residue,

to the isopropyl alcohol or otherwise disposed of.

The purified cyclohexane product is then discharged through line 23.

When the distillation has been continued to a point at which the most of the cyclohexane has been taken over as distillate, or the ratio of benzene to cyclohexane in the distillate going over to tank I! has risen to about that in the cyclohexane-benzene mixture supplied to still I, the distillation may be stopped and distillation residue withdrawn through outlet I. If the amount of isopropyl alcohol employed has been such that it is substantially completely distilled out of still i, this residue will, of course, require no treatment for separation of the isopropyl alcohol. 0n the other hand, if excess isopropyl alcohol has been employed, the residue may be washed with water to effect its removal. The oilalcohol mixture in tank i8 is then returned via pipe 24 to still i. The balance of the charge in still I is made up by adding additional cyclohexane-benzene mixturev and isopropyl alcohol. The system is now ready to commence a second distillation.

A cyclohexane-benzene mixture initially containing 30 parts of benzene and 70 parts of cyclohexane by volume. was distilled by the process described above with isopropyl alcohol in amount largely in excess of the ratio 2 volumes of the alcohol to 3 volumes of the cyclohexane in the A rectifying column equivalent to 30 theoretical plates and a reflux ratio between 15 and 20 to 1 (i. e. returning 15 to 20 parts of liquid through line I; for each 1 part withdrawn through line H), were employed. Approximately halfof the cyclohexane in the mixture was recovered as substantially pure cyclohexane (i. e. containing less than 2% benzene).

By continuing the distillation at the same reflux ratio, a second cut containing on the order of 20% benzene and 80% cyclohexane (alcoholfree basis) may be distilled off into tank II for return with a succeeding batch. Somewhat more than one-third of the benzene and cyclohexane of the original mixture may be collected in this in- The distillation residue, containing on the order of 3 parts by volume of benzene for each 2 parts by volume of cyclohexane may be withdrawn and subjected to any For example, after the hydrocarbon mixture may be subjected to an azeotropic distillation with methanol or other suitable azeotropic distillation agent to provide a pure benzene product and a cyclohexane con- The cyclohexane concentrate then may be used as an initial material for the production of pure cyclohexane by the present process.

An azeotrope of water and benzene boils at substantially the same temperature as the auand benzene, which comprises subjecting said mixture to distillation and rectification of the vapors under substantially anhydrous conditions in the presence 01' isopropyl alcohol in an amount corresponding to at least 2 volumes of said alcohol to every 3 volumes of cyclohexane taken over as distillate and collecting the resulting distillate which contains cyclohexane substantially free from benzene separate from any distillate which contains more than 2% benzene, on the basis of its total hydrocarbon content, distilled from said mixture.

2. The method of producing cyclohexane substantially tree from benzene from a hydrocarbon mixture substantiallyconsisting o1 cyclohexane and benzene in which the cyclohexane constitutes a major proportion by volume and the benzene a minor proportion by volume of the mixture, which comprises subjecting said mixture to distillation and rectification of the distilled vapors under substantially anhydrous conditions in the presence 01. isopropyl alcohol in an amount corresponding to at least 2 volumes oi' said alcohol to every 3 volumes of cyclohexane taken over as distillate and collecting the resulting distillate which contains cyclohexane substantially free from benzene separate from any distillate which contains more than 2% benzene, on the basis of its total hydrocarbon content, distilled from said mixture.

3. The method of producing cyclohexane substantially free from benzene from a cyclohexanebenzene mixture obtained by the catalytic hydrogenation of benzene in which the cyclohexane constitutes a major proportion by volume and the benzene constitutes a minor proportion by vol-' ume oi. the mixture, which comprises subjecting said mixture to distillation and rectification of the vapors under substantially anhydrous conditions in the presence of isopropyl alcohol in an amount corresponding to at least 2 volumes of said alcohol to every 3 volumes of cyclohexane taken over as distillate and collecting the result-' ing distillate which contains cyclohexane substantially free from benzene separate from any distillate which contains more than 2% benzene, on the basis of its total hydrocarbon content, distilled from said mixture.

MARSTON L. HAMLIN. 

